1. Field of the Invention
This invention relates to the field of lasers and in particular to the pump sources used in lasers.
2. Description of Related Art
Lasers work by creating a so-called population inversion in a material (conventionally referred to as the “gain medium”) which is then stimulated to produce a coherent emission of photons. The pump source is the component of the laser which excites the gain material into a state of population inversion and different lasers utilise a variety of processes to create the population inversion. For example, the common He—Ne laser uses a high voltage electrical discharge to excite a He—Ne mixture in a discharge tube to population inversion.
Typical flashlamp pumped dye lasers comprise several linear flashlamps which are symmetrically placed around a central cell containing dye (the gain medium). A reflector surrounds both the dye cell and flashlamps and directs light into the dye cell (see L. G. Nair, ‘Dye lasers’, Prog.Quant.Electr., vol. 7, pp153-268, 1982 for a discussion on flashlamp pumped dye laser structures). The most common dye material used is rhodamine 6G which has been dissolved in methanol solvent. Generally powerful, incoherent pump sources are used in order to obtain efficient system operation from a dye laser. The most common example is that of a dye laser being pumped by a conventional xenon filled flashlamp. Such a device can operate with efficiencies of >1% although a level of 0.5 to 1% is more typical.
Using the above type of flashlamp as a pump source has a number of drawbacks. Firstly, there is the risk of a flashlamp explosion. When a discharge occurs through a flashlamp a plasma filament is heated electrically and expands rapidly. This creates a shockwave which can rupture the surrounding silica envelope and thereby causing a flashlamp explosion. This risk of damage means that the level of electrical energy that can be used to drive the flashlamp must be limited. This has important consequences on the laser efficiency and on the output energy of a flashlamp pumped laser.
Secondly, the optical spectrum from a flashlamp is thermal in nature and can be considered a quasi black body with a temperature of ˜20000 K. This means that a considerable fraction of the pump light resides in the hard ultra-violet region of the electromagnetic spectrum and this causes significant degradation and heating of the dye solution, as well as resulting in a poor absorption spectrum overlap (The dye solution that comprises the gain material consists of a dye and a solvent. Hard ultra-violet radiation from the pump source will damage the dye molecules making them unable to take part in laser action. Also hard UV photons are very energetic so if they are absorbed by the dye they will be converted into a large amount of heat which is dissipated in the dye solution.). Often in smaller flashlamps ablation of the flashlamp itself can produce materials that contaminate the Xe gas thereby affecting the emission profile.
Thirdly, flashlamps generally have a high initial impedance which limits the energy deposition rate for the discharge. This results in a relatively long electrical driving pulse (of the order of ˜μs) and can promote the development of significant laser losses such as large triplet state population and/or powerful thermal lensing. The triplet state is a significant loss mechanism for dye lasers. When a dye molecule is in the triplet state it is unavailable for laser action and is a source of absorption losses for the laser emission. A high triplet state population therefore reduces laser power and energy. Thermal lensing in the dye gain medium causes a deterioration of the laser beam quality and is a loss mechanism for laser emission. Thermal lensing is caused when heat is deposited in the dye gain medium which causes refractive index changes in the solvent. The solvent therefore has the properties of a lens.
Surface discharge pumping mitigates at least some of the aforementioned disadvantages and is described for example in Optics Letters 20 pp 1011-1013 and the Journal of Physics D (Applied Physics) 29 pp 2806-2810 (1996) in relation to pumping of iodine and HF lasers, respectively.